There are three major sub-types of GABA receptors in the central nervous system (CNS): GABAA, GABAB and GABAC receptors. The pharmacology of GABAA and GABAB receptors has been extensively investigated. GABAC receptors are the most recently described sub-type of GABA receptors, and are therefore the least studied sub-type.
γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system (CNS) and activates the three major sub-types of GABA receptors in the central nervous system (CNS), the GABAA, GABAB and GABAC receptors. GABAA receptors are ligand gated Cl−ion channels which are inhibited by the alkaloid bicuculline (Johnston, 1996a). GABAA receptors are heterooligomeric made up of α, β, γ, δ and θ subunits. GABAB receptors are transmembrane receptors coupled to second membrane messenger systems and Ca+ and K+ Channels via G-proteins. These receptors are not blocked by bicuculline, but are activated by (−)-baclophen and 3-aminopropylphosphinic acid (CGP27492) and blocked by saclofen (Kerr and Ong, 1995).
GABAC receptors (sometimes called GABANANB or ρ receptors) were first proposed when a series of conformationally restricted GABA analogues, including cis-4-aminocrotonic acid (CACA), that had bicuculline insensitive depression actions on neuronal activity, showed no affinity for [3H]baclofen binding sites in rat cerebellar membranes (Drew et al, 1984). GABAC receptors with similar pharmacology were first found in neurons in rat retina (Feigenspan et al, 1993) and white perch retina (Qian et al, 1993).
Three major sub-types of GABAC receptors are now known, namely ρ1, ρ2 and ρ3 (Chebib and Johnston, 2000). GABA is a flexible compound, due to its rotation about the C2-C3 and C3-C4 bonds. It can exist in a range of low energy conformations (Johnston et al, 1978). Analogues of GABA in which two of these conformations have been restricted by the introduction of unsaturation in the form of a double bond at the C2-C3 position have been prepared. Two compounds that represent these restricted conformations are CACA and trans-4-aminocrotonic acid (TACA) (Johnston et al, 1975). CACA and TACA have fewer degrees of rotational freedom than GABA, and can only rotate about the C3-C4 bond (Johnston et al, 1978). CACA is a partially folded analogue of GABA. It has moderate activity at GABAC receptors expressed in Xenopus oocytes, and although its agonist activity is weak, it is to date the most selective agonist at these receptors, having minimal activity on GABAA and GABAB receptors (Johnston, 1996b). TACA is an extended analogue of GABA. It has potent agonist activity at GABAC receptors expressed in Xenopus oocytes; however, it is not selective, as it is also a potent GABAA receptor agonist (Johnston, 1996b).
Woodward et al (1993) tested many GABA analogues on poly (A)+ RNA from mammalian retina expressed in Xenopus oocytes to determine a pharmaceutical profile for GABAC receptors. From this study, it was found that phosphinic and methylphosphinic acid analogues of GABA, which are known to be potent GABAB receptor agonists, were potent antagonists at GABAC receptors. Several straight chain phosphinic, methylphosphinic and phosphonic acid analogues of GABA were shown to be potent antagonists at GABAC receptors, including (3-aminopropyl)methylphosphinc acid (CGP35024, KB=0.8 μM), 3-aminoporpylphosphinic acid (CGP27492, KB=0.8 μM) and 3-aminopropylphosphonic acid (3-APA, KB=1.8 μM) (Woodward et al, 1993). These agents are not selective for GABAC receptors, as CGP35024 and CGP27492 are also very potent GABAB receptor agonists, while 3-APA is a GABAB receptor agonist.
To date, the most potent and selective GABAC receptor antagonist known is 1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid (TPMPA, KB=2.1 μM) (Murata et al, 1996; Ragozzinno et al, 1996). TPMPA produces 50% inhibition of GABAC receptor activation at 2.1 μM.
Chebib et al, 1997 demonstrated that TPMPA, the phosphinic and methyl phosphinic acid analogues of CACA and the closely related analogue TACA, and 3-aminopropyl-n-butyl-phosphinic acid (CGP36742), an orally active GABAB receptor antagonist, are GABAC receptor antagonists.
It has been shown that GABAC receptor antagonists have therapeutic application.
Froestl et al., 1995 investigated a series of GABAB receptor antagonists in various memory and learning tests in rats and mice. Only one compound in this series, 3-aminopropyl-n-butyl-phosphinic acid (CGP36742), reversed age related deficits of old rats. The cognition-enhancing effects of this compound were confirmed in learning experiments in monkeys (Froestl et al., 1995).
The cognition-enhancing effects of CGP36742 is not satisfactorily explained by its GABAB antagonist properties, since much more potent GABAB antagonists have been described that do not have these effects. Some of the present inventors have previously shown that CGP36742 has similar potency as a GABAC antagonist to its potency as a GABAB antagonist (50% inhibition of receptor activation being found at 38 μM and 62 μM against GABAB and GABAC receptors respectively) (Chebib et al, 1997). None of the other potent GABAB antagonists used in Froestl et al., 1995 showed activity against GABAC receptors. These findings indicate a role for GABAC receptor antagonism in the cognition-enhancing properties of CGP36742.
WO 98/58939 describes a method of enhancing the cognitive activity of an animal in need of such treatment, comprising the step of administering an effective amount of a compound which has GABAC receptor antagonist activity to said animal. That document also describes a method of stimulating memory capacity, comprising the step of administering an effective amount of a compound which has GABAC receptor antagonist activity to an animal in need of such treatment. The document also describes novel compounds having GABAC receptor antagonist activity of the general formula I and general formula II as defined in that document.
Australian provisional patent application nos. P07501 and PP2985 describe various generic classes of compounds which are said to have GABAC receptor antagonist activity.
Most of the GABAC receptor antagonists described in the prior art, such as CGP36742, are non-selective, that is, they also have significant activity at the GABAA or GABAB receptors. The use of such compounds as GABAC receptor antagonists may have undesirable side-effects via their action at the GABAA and/or GABAB receptors.
It would be desirable to identify selective antagonists of the GABAC receptors. Such agents are needed to determine the physiological role of GABAC receptors and to provide therapeutic agents with a lower risk of unwanted side-effects due to actions at the GABAA and/or GABAB receptors.